CODEWITHSUNDEEP
haskell
user4329496
user4329496

Reputation:

Noob having difficulties with Haskell - Not in scope errors

I am a beginner with Haskell and having some difficulties getting a tuple consisting of 4 empty lists to work. The below code is the entirety of my Haskell Program.

import Data.List
import System.IO

l = []
h = []
a = []
x = []

TextEditor = (l, h, a, x) 

backspace :: TextEditor -> TextEditor
backspace (TextEditor l h a x) = 
    (TextEditor(reverse (tail (reverse l))) [] a x)

I get multiple errors.

Not in scope: data constructor ‘TextEditor’
Not in scope: type constructor 'TextEditor'

Despite googling I can't work out what's wrong with my functions. Could someone help push me in the right direction please?

Upvotes: 0

Views: 107

Answers (2)

leftaroundabout
leftaroundabout

Reputation: 120751

I guess what you're trying to do is this:

type L = [Char]  -- aka String
type H = [Char]
type A = [Char]
type X = [Char]

data TextEditor = TextEditor L H A X -- You really should use more discriptive names

backspace :: TextEditor -> TextEditor
backspace (TextEditor l h a x) = 
    (TextEditor(reverse (tail (reverse l))) [] a x)

Upvotes: 2

Jules
Jules

Reputation: 14520

What you've done here is declared a top-level scoped symbol (like a variable) called TextEditor.

What you probably want to do is to declare a TextEditor data type and its corresponding type constructor, which might be done like this:

data TextEditor = TextEditor ([String], [String], [String], [String])

(Your definition may vary; you didn't declare the types of l, h, a or x so I'm just assuming [String])

I'd recommend reading up about data declarations and typeclass definitions in the appropriate LYAH chapter.

Upvotes: 1

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